What causes depression? The honest version of the list.

Every article promises the causes of depression ranked by how common they are. That list does not exist, and the numbers in it are invented. Here is what the evidence actually supports, and what is worth ruling out first.

Search for the causes of depression and you will find a tidy numbered list. Chemical imbalance, then genetics, then stress, then diet, often with percentages attached.

Almost none of it is real. Not because the items are wrong, but because the ranking is invented. There is no study that tells you what share of depression is caused by what, and anyone presenting one is ordering their own guesses.

That sounds like a dead end. It is not, because two honest lists do exist, and between them they answer the question people are actually asking.

Why the ranked list cannot exist

Depression is not one illness with one mechanism. It is a label for a pattern of symptoms that can be arrived at from many directions, and most people arrive by several at once.

Consider someone who is isolated, sleeping badly, in pain from an untreated condition, and carries a family history. Which one caused it? The question has no answer, because the causes are not separable and they feed each other. Bad sleep deepens pain, pain deepens isolation, isolation worsens sleep.

To produce a ranked list you would need to intervene on one factor at a time, in a large population, and watch what happens. For most of these, that study would be unethical or impossible. So nobody has done it, and nobody can.

What we have instead is risk: how much more likely depression becomes when a factor is present. That is a real number, and it is not the same as a cause.

The list that does exist: risk, with numbers attached

Genetics. The best meta-analysis of twin and family studies puts the heritability of major depression at 37 percent (Sullivan 2000, 95% CI 31 to 42). Worth reading carefully. It does not mean 37 percent of your depression is genetic. It means that across a population, roughly that share of the *variation* in who gets depressed tracks genetic differences. Most of the variation does not.

Childhood adversity. In the original adverse childhood experiences study, adults reporting four or more categories of childhood adversity had four to twelve times the risk of alcoholism, drug use and depression compared with those reporting none (Felitti 1998, 13,494 adults). This is one of the largest and most consistent signals in the field, and it is a dose-response relationship, not a yes or no.

Insomnia. People with insomnia are 2.6 times more likely to develop depression later (Baglioni 2011, meta-analysis of longitudinal studies). The direction matters: this is insomnia at baseline predicting depression at follow-up, so it is not simply depression causing bad sleep. It runs both ways.

Isolation, chronic illness, chronic pain, and major life events all raise risk substantially and are harder to put a single clean number on, because they are measured differently in every study.

Notice what is missing from that list: a neurotransmitter.

The chemical imbalance question, answered properly

The serotonin theory of depression has little support. The umbrella review that made this public in 2022 found no consistent evidence that depression is caused by low serotonin (Moncrieff 2022).

So "find out which chemical you are low on" is not a thing that can be done. There is no blood test, no scan, no panel. If a clinic offers you one, that is a commercial product, not medicine.

This is also why the question "is it biological or psychological" does not survive contact with the evidence. Sleep is biological and responds to behavior. Isolation is social and changes the body. The distinction is a hangover from an older argument and it does not help you decide what to do on Monday.

The other list that does exist: what to rule out

This is the practical one, and it is the reason the question is worth asking at all.

A minority of people presenting with depression have a medical condition producing the symptoms, or making them worse. These are cheap to test, they are frequently missed in a ten minute appointment, and finding one changes the treatment completely.

Thyroid. An underactive thyroid produces fatigue, low mood, slowed thinking and weight change. It imitates depression closely enough to be missed for years. Ask for TSH by name.

Iron. Ask for ferritin, not just a full blood count. Hemoglobin can read normal while your iron stores are empty, and the fatigue arrives before the anemia does.

B12 and vitamin D. Both deficiencies produce low mood and exhaustion. Both are common, cheap to measure, and simple to correct when genuinely low. Correcting them when they are not low does nothing.

Sleep apnea. Fragmented sleep every night produces something indistinguishable from depression, and the person often has no idea their sleep is broken. If you snore and wake unrefreshed, say so.

Medication side effects. Some common prescriptions list low mood as an effect. Worth reviewing what you already take before adding anything.

Perimenopause. Mood symptoms often arrive before the obvious signs and get treated as primary depression.

None of these diagnose depression. They rule out the things that imitate it.

What to do with all this

Get the cheap tests done first. Not because they will probably find something, but because when they do, it changes everything, and the test costs almost nothing.

Stop looking for the cause. For most people there isn't one. Looking for it is a trap that delays doing anything, which is itself one of the things that keeps depression going.

Work on what is both modifiable and well evidenced, which usually means sleep, movement, contact with people, and treatment. The risk factor list is more useful read backwards: it tells you where the leverage is.

If you want a single sentence to take away: you do not need to know what caused it to treat it, and waiting until you do is how people lose years.

This article is for general information and is not a substitute for professional care. If you are in crisis or thinking about harming yourself, please contact your local emergency services or a crisis line. See crisis lines.